Throughout the year, birders look forward to changing seasons and avian scenes as they explore woods, grasslands, and wetlands: the spectacle of spring migration, the songs of breeding birds, the autumn southward flight of wintering species from northern nesting grounds.

Increasingly, though, both casual bird-watchers and ornithologists note a steady decline in numbers—not just of endangered species, but also of common birds not usually considered to be at risk. Study after study, survey after survey show a worrisome downward trend in populations.

Migrants return from Central America to find that the brushy field where they nested the previous year is now a strip mall.

Millions of songbirds annually suffer bloody death in the claws of domestic cats. Millions more collide with city skyscrapers or communications towers, or fly into the glass windows of suburban houses.

And climate change could degrade or even eliminate habitats in ways that scientists have only recently begun to study and try to forecast.

The report, based on 21 studies of cats and birds in the United States and Europe, showed that cat predation may well be the greatest source of human-related bird mortality in the country. (Read "Why Novelist Jonathan Franzen Loves Birds.")

The American Bird Conservancy addresses this often-contentious issue in its Cats Indoors campaign, aimed at convincing cat owners and local lawmakers that the environment is better off when cats are kept inside—as are cats themselves.

Though it brings the subject of bird conservation to a wide audience, the attention-grabbing news about cat predation reflects only one of many dangers looming for the continent's bird life, some far more ominous.

Though numbers are subject to debate, one biologist estimated in 2009 that 440,000 birds were dying each year through impact with wind turbines. Whatever the figure, it's bound to rise as more wind farms are constructed.

"We recognize and support the use of renewable energy to avoid climate change, which will impact everything," says Gary Langham, vice president and chief scientist for the National Audubon Society. "Obviously we're concerned about getting the siting done correctly, so there's minimal chance of killing a condor or a golden eagle."

The American Bird Conservancy and other groups support proposed regulations that would keep wind turbines away from migration routes, wetlands, wildlife refuges, and similar areas likely to be frequented by birds.

"In a way, this is the focal area at the moment because it's the one that's changing fastest, with the greatest increase in threat," says conservancy vice president Michael Parr. "If we could get it right now, this will set the course for the next 30 years or so."

In a related move, a coalition that includes conservationists, the Federal Communications Commission, and the Federal Aviation Administration has been studying the design of communications towers to try to reduce the estimated six-million-plus birds killed each year in collisions with the towers and their guy wires (the heavy cables that anchor the masts to the ground).

The most recent findings show that the steadily glowing red lights seem to confuse flying birds, causing them to crash into towers or to fly in circles until they drop from exhaustion. Replacing these lights with flashing strobe lights could cut bird mortality by as much as 75 percent without compromising aircraft safety.

Tower owners have questioned the accuracy of bird-death figures, and resist the expense of converting lights on existing structures. Nonetheless, conservation groups continue to push for safer tower lights, and the FAA is studying proposed new regulations.

Acre by Acre, a Losing Battle

Meanwhile, many ornithologists, while regretting bird deaths caused by cats and towers and supporting efforts to reduce them, see those and similar issues as distractions from a far more important problem.

"To me, the top three threats to birds overall are habitat loss, habitat loss, and habitat loss," says Ken Rosenberg of the Cornell Lab of Ornithology. "We're losing the battle acre by acre."

Gary Langham of Audubon agrees. "Certainly to this point, loss of habitat is the number one problem," he says. "In some cases, say in California, we have removed or converted up to 99 percent of riparian [streamside] habitat and 95 percent of wetlands. Those losses have huge impacts on birds."

Grassland birds, for example, have declined about 40 percent in the past 40 years, reflecting the continuing loss and degradation of native prairie through expansion of cropland, overgrazing, and invasion by alien vegetation.

Of the more than 300 million acres of grasslands and pastures across the United States, only about 13 percent is publicly owned. As a result, conservation of such habitats depends largely on incentives to private landowners, including the federal Conservation Reserve Program (CRP), which pays farmers to take land out of agricultural production and convert it to environmentally valuable uses.

"The biggest factor in agricultural systems is changing commodity prices," Cornell's Rosenberg says. "With this big push to raise corn for ethanol production, just since 2008 we've lost 23 million acres that were in CRP and other farm-bill programs and have been converted back to crop production.

"So you could think of it this way: While we're arguing about wind towers and all these other issues, we've lost 23 million acres of habitat. That's the kind of really big thing that can have a major effect on bird populations."

Just the Right Habitat

Habitat loss doesn't necessarily mean acts as overt as turning forests into subdivisions or prairies into cornfields. For years, ornithologists have been worried about the cerulean warbler, a small blue-and-white bird of the eastern woodlands. Studies have shown a population decline for the species in recent decades of 50 to nearly 80 percent in some areas.

Biologists long thought that ceruleans needed mature deciduous forest, and were puzzled when the birds didn't breed in what seemed like good habitat.

But thanks to newer research, they now know that ceruleans need broken mature forest, with gaps in the canopy—a condition not present in even-aged woodland created by modern forestry practices. Modify a mature forest to create gaps, and ceruleans will return.

Such results are good news for a threatened species, but it's only one part of the puzzle that must be complete if the bird's future is to be secured.

The cerulean warbler is classified as a neotropical migrant, one of dozens of birds that nest in North America but spend the winter in the tropics. They need habitats that provide food and shelter year-round, not just on North American breeding grounds.

But southern shelter is harder to come by as development degrades the environment in areas from Mexico and the Caribbean to Amazonia. The cerulean warbler spends winter on the slopes of South America's Andes, at the same elevation where coca is grown to supply cocaine for illegal drug markets in the United States and Europe.

Forest loss for coca cultivation (and possibly spraying of herbicide in antidrug operations) may be harming cerulean populations nearly as much as changes to their habitats in North America.

Even if habitat isn't destroyed, it can still be "lost" to birds in other ways. The piping plover, a small brown-and-white shorebird, nests on the same seaside beaches, lakeshores, and river sandbars that humans use for swimming, picnicking, and driving all-terrain vehicles.

The population of piping plovers dropped to fewer than 3,000 breeding pairs in 2001. Recovery efforts have been under way for decades, including temporarily fencing off nesting beaches during breeding season and controlling predators.

Results have been mixed, with plovers in a few regions doing well while others barely hold their own. Measures to protect piping plovers have angered residents of towns such as Seabrook, New Hampshire, and Plymouth, Massachusetts, who see beach closures as violating their rights to use public recreation areas.

"Most Serious Threat This Century"

It's easy to see the sudden loss of habitat when, say, a marsh is drained. The effects of climate change, by comparison, move in extreme slow motion. Yet scientists say the results will be far more profound.

The Bicknell's thrush is a rare bird found in the United States only in high-elevation spruce-fir forests on a few mountaintops in the Northeast. In Canada, it breeds in similar habitat at lower elevations.

The great majority of these brown, robin-sized birds winter on the Caribbean island of Hispaniola. Biologists worry about the future of this vulnerable species, as it faces threats at both ends of its migratory journey.

If global temperatures continue to rise, spruce-fir forest could disappear from peaks in the United States, displaced by hardwood forest rising along mountain slopes. That would mean the end of Bicknell's thrush as a breeding species in this country. (It could take longer for the species to be seriously affected in Canada.)

More threatening, however, is the condition of the wintering range on Hispaniola.

Decades of intensive logging and clearing for agriculture have destroyed nearly all natural forest in the desperately poor country of Haiti, which comprises the western third of the island. In the Dominican Republic, which occupies the rest, illegal logging is eating away at a national park that's one of the most important winter refuges for the Bicknell's thrush.

The species could well disappear even before climate change eliminates its nesting habitat.

But climate change will affect birds in multiple ways, some of them impossible to predict. One small example: Gray jays, the bold "camp robbers" of boreal forests, depend for much of their winter food on nuts and other items they cache during fall, when food is abundant. If temperatures rise, much of that food could rot by the time the jays need it, instead of being safely stored in a natural freezer.

The National Wildlife Federation recently issued a report calling climate change "the most serious threat this century facing America's migratory birds."

Changing precipitation patterns in the midwestern "prairie pothole" region, the NWF says, could cause the loss of significant breeding habitat for ducks, including mallards and pintails.

Another potential climate change victim: the shorebird called red knot, which depends on horseshoe crab eggs in Delaware Bay to provide energy on its northward spring migration. Changing climate, the report says, could disrupt the timing of horseshoe crab egg-laying and the red knot's stopover in the bay.

In one significant way, North American birds are better off than their relatives across the Atlantic. As Jonathan Franzen reports in the July issue of National Geographic, hunting takes a significant toll on a wide array of birds in Europe and northern Africa—not just waterfowl and other traditional game species, but also songbirds, hawks, and shorebirds, killed by the millions in nearly unregulated slaughter.

Thanks to better (and better-enforced) conservation laws and different hunting traditions, North American nongame birds don't suffer the same year-round ordeal of guns and traps; our migratory species aren't forced to run a gantlet of indiscriminate shooters as they travel to and from their breeding and wintering grounds.

The threats they encounter here, though, are varied and—whether immediate or long-term—just as deadly.